1. Field of the Invention
This invention relates to a process for producing reduced iron, and more particularly to a direct reduction process to produce reduced iron efficiently and economically by directly utilizing methane as a reducing gas.
2. Description of the Prior Art
The direct reduction process has undergone various technical improvements and it is only in recent years that large scale commercial plants employing this process are beginning to be constructed. There are various types of direct reduction processes, such as the rotary-kiln process, the static bed process, the fluidized bed process, and the shaft furnace process. Among these processes, the shaft furnace process is based on a counter current principle, whereby iron oxide raw materials are charged from the top of the furnace into contact with a reducing gas blown in from the bottom of the furnace so as to produce reduced iron, and this process is gaining ground because of its high thermal efficiency and gas utilization. As the reducing gas for the direct reduction process, a gas which consists mainly of carbon monoxoide (CO) and hydrogen (H.sub.2) has been used. This reducing gas is produced by a so-called gas reforming technique, by which hydrocarbon (CnHm) such as methane (CH.sub.4) reacts with water vapor and carbon dioxide (CO.sub.2) or oxygen (O.sub.2) to form CO and H.sub.2. Accordingly, it is necessary to install in the iron-production facility a reformer for decomposing CH.sub.4 into CO and H.sub.2, in addition to a reactor where iron oxides such as iron ores or pellets are reduced into metallized iron. Furthermore, CH.sub.4 has not been positively utilized as a direct reducing gas. The reasons why CH.sub.4 has not been directly used as reducing gas and why the reforming process has been required, are as follows.
(1) At customary temperatures for the reducing reaction (ordinarily below 900.degree. C.) in furnaces such as the shaft furnace, the potential of CH.sub.4 for reducing the iron oxide is not sufficient. Although the potential can be fairly improved by raising the reaction temperature, the high temperature results in so-called clustering, a phenomenon in which reduced iron pellets stick to each other, obstructing the descent of the burden; hence the temperature cannot be raised to more than the normal reaction temperature.
(2) As temperature rises, CH.sub.4 decomposes into hydrogen and solid carbon according to the following reaction. EQU CH.sub.4 =2H.sub.2 +C (I)
And at the same time reduced iron promotes this reaction as a catalyst. When this reaction proceeds further, deposited carbon formed by this reaction fills the voids within the burden, resulting in a higher gas pressure drop and thereby smooth operation will be prevented.
(3) The reaction in which the iron oxide is reduced with CH.sub.4 is expressed as follows: EQU Fe.sub.2 O.sub.3 +CH.sub.4 =2FeO+CO+2H.sub.2 (II) EQU FeO+CH.sub.4 =Fe+CO+2H.sub.2 (III)
Both of these reactions are highly endothermic reactions, and it is difficult to compensate for the heat required for these reactions in the ordinary shaft furnace process and therefore smooth operation cannot be maintained.
For the reasons described above, CH.sub.4 has not been utilized as a reducing gas for direct reduction, making it imperative that the gas reforming process be employed. However, this reforming process greatly affects the productivity and production cost of the reduced iron. Although there are some methods now under study, in which CH.sub.4 is introduced directly into the reactor to reduce the iron oxide, or the gas is reformed while iron oxide is reduced, no satisfactory results have yet been obtained. The only example that directly uses CH.sub.4 for reducing iron oxide may be found in the Japanese Patent Publication No. 52-22612, in which only a limited amount of CH.sub.4 is directly used as a reducing agent making use of the auto-catalytic reaction of the metallized iron.